The Physical Mechanisms of Surface Photoreactions.

The study of photoreactions of molecules adsorbed on solid surfaces in ultrahigh vacuum has attracted increasing attention over the last several years and will undoubtedly develop as one of the major fields in surface science in the coming years. In this thesis, photodesorption, photodissociation, and photosynthesis of molecules on semiconductor and metal surfaces are characterized by mass spectrometry and vibrational spectroscopy, and the physical mechanisms that drive surface photoreactions are elucidated by analyzing the photoyield. The surface-temperature rise due to unfocused continuous-wave irradiation is experimentally measurable and fully controllable. It is found that desorption of Mo(CO)_6 and CH_3 CH_2OH from Si(111)7 x 7 under high-power visible irradiation is due to irradiative heating. The thermal reaction channel, however, is effectively suppressed at relatively low powers. Mo(CO)_6 photodissociates nonthermally on Si(111)7 x 7 and Cu(111) under UV irradiation. The surface photodissociation is induced by direct photoelectronic excitation of the adsorbate and differs from the gas-phase photodissociation by reduced photoyields and incomplete dissociation. NO molecules adsorbed on Si(111)7 x 7 have both occupied and empty levels very close to the Fermi level of the substrate. It is found that hot carriers photogenerated in the substrate with UV to IR photons can interact with these levels, indirectly leading to photoexcitation of NO. Simultaneous photodesorption of NO, photodissociation of NO, and photosynthesis of N_2O from NO occur on Si(111)7 x 7. The occupied and empty levels of Mo(CO) _6 are far from the Fermi levels of Si(111)7 x 7 and Cu(111), which explains the absence of the substrate -mediated photoexcitation. However, when K is coadsorbed with Mo(CO)_6 on these surfaces, the Fermi levels are raised substantially towards the empty levels of Mo(CO)_6. Consequently, the interaction of photogenerated hot electrons with these empty levels becomes energetically possible. Due to this additional channel, photodissociation of Mo(CO) _6 on K-preadsorbed surfaces is substantially enhanced in the UV and extends to the visible and near -IR regimes.